By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 14, 2026
Absolute pressure is measured from a perfect vacuum. Gauge pressure is measured from the local atmospheric pressure. The two scales differ by exactly one atmosphere: Pabs = Pgauge + Patm, which is about 101.325 kPa (14.7 psi) at sea level.
That one sentence settles most homework questions. On a real transmitter order it is only the start, because the atmosphere is not a fixed number. It falls with altitude, it moves with the weather, and whether that movement lands in your reading depends on which reference you specified. Pick the wrong one on a vacuum job or a leak test and the instrument reports the barometer, not the process.
This guide gives the two definitions, the conversion with worked numbers, an altitude table you can size against, and a selection table for vented gauge, sealed gauge, and absolute references.
Contents
- The two reference points
- Absolute pressure
- Gauge pressure
- Converting between absolute and gauge pressure
- Atmospheric pressure is a moving zero
- Vented gauge, sealed gauge, or absolute
- When to use each reference
- Measuring instruments for each reference
- FAQ
The two reference points
Every pressure reading is a comparison against a reference. There are only two references that matter in process work: a vacuum, and the air around the instrument.
| Property | Absolute pressure | Gauge pressure |
|---|---|---|
| Zero point | Perfect vacuum (no molecules) | Local atmospheric pressure |
| Can it read negative? | No, never | Yes, below atmosphere (vacuum) |
| Unit suffix | psia, kPa abs, bar a | psig, kPa g, bar g |
| Moves with weather or altitude? | No | Yes, the zero shifts with the barometer |
| Typical services | Vacuum, leak testing, gas density, altimetry | Hydraulics, pneumatics, tank level, pump discharge |
A reading of 0 psig and a reading of 14.7 psia describe the same physical state at sea level: an open pipe.
Absolute pressure
Absolute pressure counts every bit of pressure present, starting from nothing at all. A sensor built for it holds a small evacuated chamber behind the diaphragm, so the cell compares the process against a vacuum that never changes. The reading is always positive and it means the same thing in Shanghai, in Denver, and in a rising storm.
That independence is the whole point. When a calculation needs the true state of a gas, density from the ideal gas law, boiling point under vacuum, compressor performance, it needs absolute pressure. The same goes for any measurement that must not move when the weather does, which is why absolute pressure transmitters carry the vacuum and leak-test services in a plant.
Gauge pressure
Gauge pressure is the difference between the process and the air outside the instrument. A standard gauge sensor keeps the back of its diaphragm open to ambient air through a small vent, usually a tube running inside the signal cable. Whatever the barometer does, it does to both sides of the diaphragm at once, so the reading tracks the process alone.
Gauge readings come in three signs. Positive means above atmosphere: a pump discharge at 250 kPa g. Zero means an open system. Negative means below atmosphere: a condenser pulling −21 kPa g of vacuum. That negative range is real vacuum measurement, and plenty of compound-range transmitters cover both sides of zero.
Most process instruments ship as gauge reference for a practical reason: the equipment around them, relief valves, pump curves, pipe ratings, is all specified in gauge terms. A vessel rated 10 bar g is rated against the atmosphere pushing back on its shell, not against a vacuum.
Converting between absolute and gauge pressure
One formula covers both directions:
Pabs = Pgauge + Patm and therefore Pgauge = Pabs − Patm
Use the atmospheric pressure at your site, not the sea-level constant. Three worked numbers, all at standard sea-level atmosphere (101.325 kPa = 14.696 psi):
- A pump discharge reading 250 kPa g is 250 + 101.3 = 351.3 kPa abs.
- An air receiver at 100 psig is 100 + 14.7 = 114.7 psia.
- A dryer running at 80 kPa abs reads 80 − 101.3 = −21.3 kPa g on a gauge-reference instrument: a vacuum.
For quick two-way conversion with the altitude correction built in, use our absolute and gauge pressure calculator. If the units themselves are the question, psi to bar to kPa, the pressure units guide has the full conversion tables.
Atmospheric pressure is a moving zero
The term Patm in the formula is not 101.325 kPa unless you are at sea level on a standard day. Two things move it.
Altitude. The standard atmosphere (ISO 2533) thins fast with elevation:
| Altitude | Atmospheric pressure | In psi |
|---|---|---|
| Sea level | 101.3 kPa | 14.70 |
| 500 m (1,640 ft) | 95.5 kPa | 13.85 |
| 1,000 m (3,280 ft) | 89.9 kPa | 13.04 |
| 1,600 m (Denver) | 83.5 kPa | 12.11 |
| 2,000 m (6,560 ft) | 79.5 kPa | 11.53 |
| 3,000 m (9,840 ft) | 70.1 kPa | 10.17 |
| 4,000 m (13,120 ft) | 61.6 kPa | 8.94 |
ISO 2533 standard atmosphere. Weather adds roughly ±3 kPa on top of any row.
Weather. A passing front moves the barometer by up to about ±3 kPa (±30 mbar) at a fixed site. A vented gauge instrument rides on top of that automatically. An absolute instrument does not, which cuts both ways.
Here is the error that decides the selection. Suppose a task actually needs absolute pressure, holding a coating chamber at 150 kPa abs, but the loop uses a transmitter with a 0 to 100 kPa gauge span, zeroed once at commissioning. Every 3 kPa the weather moves shows up as 3 percent of that span in false process change. The same 3 kPa against a 40 MPa hydraulic span is 0.0075 percent: invisible. Low spans near atmosphere feel the reference choice; high spans do not.
Vented gauge, sealed gauge, or absolute
Mechanically the three references are just three ways to treat the back side of the sensing diaphragm.
| Reference | Behind the diaphragm | Choose it when |
|---|---|---|
| Vented gauge | Vent tube open to ambient air | Default for most process work; open tank level; anything specified in psig or bar g |
| Sealed gauge | Chamber closed at roughly 1 atm at the factory | Vent would flood or clog: submerged sensors, washdown areas, high-pressure hydraulics above about 100 bar where the 1 atm offset is negligible |
| Absolute | Evacuated chamber, near-zero kPa abs | Vacuum processes, leak-rate testing, gas density and mass calculations, barometric compensation, any reading that must not move with weather or altitude |
Sealed gauge deserves one caution. Its trapped reference gas expands and contracts with temperature, so at low spans it drifts where a vented unit would not. Treat it as a high-pressure or submerged-service option, not a general substitute for venting.
When to use each reference
The decision usually collapses to one question: is the process itself referenced to the atmosphere, or not?
If the fluid pushes against equipment that also feels the atmosphere, relief valves, open tanks, pump casings, pipe walls, use gauge. The atmosphere cancels out of the mechanics, so it should cancel out of the measurement. If the physics of the process ignores the atmosphere, gas laws, boiling under vacuum, a sealed and evacuated volume, use absolute, because a gauge reading would smuggle the barometer into your data.
Leak testing is the classic trap. Pressurize a sealed vessel, hold it overnight, and watch the reading: on a gauge instrument a rising barometer shows up as a falling reading, indistinguishable from a small leak. The same test on an absolute transmitter shows the vessel alone. The gauge pressure transmitter is not wrong; it is answering a different question.
Application example
Refrigeration service, United States. A contractor maintaining low-temperature refrigeration systems came to us with a failed imported vacuum transmitter on the evacuation side of the plant. The replacement question was not the range; it was the reference. Evacuation targets are absolute numbers, and checking them with a gauge-reference unit would have let every weather front rewrite the readings. We matched an absolute-reference vacuum transmitter to the existing process connection and signal loop and quoted it as a drop-in replacement.
Measuring instruments for each reference
The reference is set by the sensor build, not by a menu setting, so it has to be right on the order. Our SI-3151GP gauge pressure transmitters cover vented-gauge service from 0.1 kPa spans up to 41 MPa, and the SI-3151AP absolute pressure transmitters cover the same body styles with an evacuated reference for vacuum and leak-test work. Both families sit in the wider pressure transmitter line, and the full range of sensors, switches, and gauges is under pressure instruments.
One spec-sheet habit saves rework: write the suffix every time. A range written as “0 to 10 bar” forces the factory to guess; “0 to 10 bar g” or “0 to 10 bar a” does not. The suffix conventions themselves are covered in our PSI vs PSIA vs PSIG guide.
The atmosphere is the reference this whole choice turns on. For what barometric pressure is, the units it comes in, and how it is measured, see our barometric pressure guide.
FAQ
How do you convert absolute pressure to gauge pressure?
Subtract the local atmospheric pressure: Pgauge = Pabs − Patm. At sea level that means subtracting 101.325 kPa or 14.696 psi; at altitude, subtract the value from the table above. Our absolute and gauge pressure calculator does both directions with the altitude correction included.
What is the difference between absolute pressure and total pressure?
They answer different questions. Absolute pressure names the reference point of a static measurement (zero = vacuum). Total pressure belongs to flowing fluids: it is static pressure plus the dynamic pressure of the fluid velocity, the quantity a pitot tube stagnates against. A total pressure can itself be stated in absolute or gauge terms.
What is the difference between relative pressure and absolute pressure?
Relative pressure is another name for gauge pressure, common in European documentation (and behind the “e” in the symbol Pe). It is measured relative to the atmosphere, so it moves with weather and altitude, while absolute pressure is fixed to a vacuum reference.
When should I use psia instead of PSIG?
Use psia when the atmosphere is not part of the process: vacuum work, leak-rate testing, gas density or mass-flow calculations, and any site where readings must be comparable across altitudes. Use psig for equipment that lives against the atmosphere, such as hydraulics, pneumatics, and vessel ratings. The full suffix system, including PSID, is in the PSI vs PSIA vs PSIG guide.
Specify the right reference on your next transmitter
Send us the medium, the range with its suffix, and whether the reading must hold steady against weather and altitude. We will confirm vented gauge, sealed gauge, or absolute and quote the matching build. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.